Observation of Gravitational Waves from a Binary Black Hole Merger

On September 14, 2015, the Advanced LIGO detectors in Hanford, Washington, and Livingston, Louisiana, simultaneously recorded a transient signal, designated GW150914. This signal, which swept upward in frequency from 35 to 250 Hz, represents the first direct observation of gravitational waves—ripples in the fabric of spacetime predicted by Albert Einstein a century earlier. The observation confirms the existence of binary stellar-mass black hole systems and provides a new method for testing general relativity in the strong-field, high-velocity regime.

The signal's characteristics are consistent with the final stages of a binary black hole merger. The waveform shows an inspiral phase, where two black holes orbit each other, followed by a merger and a final ringdown, where the resulting single black hole settles into a stable state. Analysis of the signal indicates that the initial black holes had masses of approximately 36 and 29 solar masses, merging to form a final black hole of 62 solar masses. The difference in mass—about 3 solar masses—was radiated away as gravitational-wave energy.

The detection was achieved using two modified Michelson interferometers, each with 4-km arms. These instruments are designed to measure minute changes in arm length caused by passing gravitational waves. The signal was identified with a matched-filter signal-to-noise ratio of 24. Statistical analysis, including a time-shift technique to estimate the background noise, determined the false alarm rate to be less than 1 event per 203,000 years, corresponding to a significance greater than 5.1 sigma. This high level of confidence confirms the event as a genuine astrophysical signal rather than instrumental noise.